Geology, ore-forming fluid and genesis of the Qiucun gold deposit: Implication for mineral exploration at Dehua prospecting region, SE China

Abstract

The Qiucun gold deposit, located in the central Fujian Province, is one of the most important gold producers at the Dehua prospecting region. The deposit is hosted in the Nanyuan Formation consisting of continental arc volcanic rocks, which belong to the Mesozoic coastal volcanic belt of south China. Gold ore bodies are controlled by a group of NE-NEE trending faults. Wall rock alteration shows that gold mineralization is closely associated with quartz (chalcedony)-illite-pyrite-calcite assemblages, in which pyrite is the dominate sulfides (>90%). Petrographic study of ore minerals shows that gold occurs as electrum grains of 2–65 μm in diameter, interstitially or in fractures within pyrite. EPMA analysis of electrum yielded compositions of 62–69 wt% Au and 28–33 wt% Ag. Fluid inclusions petrographic and microthermometric studies were carried out on auriferous quartz veins from three different elevations, i.e. 712 m, 742 m and 820 m above sea level. Two types of fluid inclusion were observed, namely two-phase liquid-rich (type I) and two-phase vapor-rich (type II) fluid inclusions. There inclusions are predominately aqueous inclusions but can sometimes contain minor CO2 and N2 as revealed by laser Raman analysis. Type I inclusions show homogenization temperatures (Th) of 181–248 °C, with salinities of 0.5–2.6 wt% NaCl equivalent. Type II inclusions display Th of 208–249 °C, with salinities of 0.5–0.9 wt% NaCl equivalent. The occurrence of platy calcite and hydrothermal breccias, together with coexisting type I and type II inclusions which shows similar homogenization temperatures but contrasting salinities, all indicate that fluid boiling is a crucial mechanism for ore deposition at the Qiucun gold deposit. Combining the geological settings, alteration, mineralization and fluid inclusion characteristics, we propose that the Qiucun gold deposit is a low-sulfidation epithermal deposit. The estimated pressure calculated by fluid inclusion boiling assemblages corresponds to a formation depth of 200 to 260 m below paleo-surface, which is slightly deeper than the current depth. Accordingly, the denudation thickness of the overlying Nanyuan volcanic rocks is supposed to be approximately 100 m since ore deposition, which is conductive to the preservation of epithermal ore deposit. It is therefore suggested that the large area of the Daiyunshan Volcanic Basin at Dehua district is of great potential for epithermal ore exploration.